An auxiliary (formal/old-fashioned) attached after a noun to indicate capacity, qualification, or the duties expected of that role — often used in expressions like '〜たる者' meaning 'one who is (in that position) should...'.
指導者たる者は模範を示さなければならない。
One who is a leader must set an example.
親たる者は子供の教育に責任を持つべきだ。
Those who are parents should take responsibility for their children's education.
専門家たる彼の意見を尊重しよう。
Let's respect the opinion of him, who is an expert.